In many applications, high density integrated circuit boards which develop a large amount of heat during operation require a large heat sink to assist in the efficient removal of the heat. As is common in the prior art, these integrated circuits are embedded in a silicon substrate circuit board. The purpose of this invention is to provide a method for removing a silicon substrate circuit board from the large aluminum heat sink needed to dissipate a large destructive heat load. In a practical case, the integrated circuit board comprised a 2.5 inch diameter flat silicon wafer substrate that is only 0.125 inches thick and very brittle. No methods for removing the wafer from the sink, without consistently damaging the wafer were known.
The silicon wafer is susceptible to chipping and cracking for a number of reasons. First, in the particular case, all circuit components were mounted on a single side utilizing surface mount technology, such as soldering and epoxy. In certain applications, due to the large amount of heat generated within a high density circuit, a gold/tin solder mix was used to ensure that the components would not come loose from the circuit board during operations. This solder has a relatively high melting point and does not enter the liquid phase until approximately 300.degree. F. No prior art methods were known, and any attempt to remove a component to check the circuit out, generally resulted in a cracked or chipped and totally useless circuit board.
The following prior art U.S. Patents were reviewed after a search:
U.S. Pat. No. 5,199,159 issued to Waldsmith; PA1 U.S. Pat. No. 5,070,603 issued to Waldsmith; PA1 U.S. Pat. No. 4,603,345 issued to Lee et al; and PA1 U.S. Pat. No. 3,837,815 issued to Bixby.
The Bixby patent relates to a method for separating a coating from the smooth surface of a base material. The temperature of the coated base metal is rapidly reduced to far below the freezing temperature of water. The coated surface is then wetted with a liquid which forms a frozen layer on the coated side of the base material. Heat is then applied to effect separation of the frozen layer from the base material, with the coating adhered to the separated frozen layer.
The Lee et al patent describes a module for a semiconductor chip, wherein the module includes a heat sink with a flat surface to which the back face of the semiconductor chip is directly bonded.
The Waldsmith patents '603 and '159 are my prior patents, and while they are relevant to cryogenic removal of components, none teaches the specific method of this invention.